Implementing loglinear classification (without training), for CL classification

appengine/findit/crash/changelist_features/min_distance.py

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from collections import namedtuple
+
+
+# N.B., this must not be infinity, else we'll start getting NaN values
+# from LinearMinDistanceFeature (and SquaredMinDistanceFeature).
+DEFAULT_MAXIMUM = 50
+
+
+# N.B., if we make this a namedtuple then it becomes horrible to inherit from.
+class MinDistanceFeature(object):
+  def __init__(self, maximum=None):
+    """
+    Args:
+      maximum (float): An upper bound on the return result. This
+        argument is optional and (effectively) defaults to infinity.
+    """
+    self._maximum = maximum
+
+  def __call__(self, result):
+    """Return the minimum ``AnalysisInfo.min_distance`` across all files.
+
+    Although this method looks like it should be a method on the
+    ``Result`` class, we have it live here in order to make coverage
+    tests happy. The downside of this is that we now have to modify
+    multiple files whenever the guts of ``Result`` change. The upside
+    is the aforementioned coverage tests, and that it helps keep the
+    ``Result`` class looking cleaner.
+
+    Args:
+      result (Result): the result to analyze.
+
+    Returns:
+      The minimum distance between (the code for) a stack frame in the
+      ``Result`` and the CL in the ``Result`` as a ``float``. If no
+      ``maximum`` is given, then we return that minimum directly. If a
+      ``maximum`` is given, then we return the smaller of it and the
+      found minimum distance.
+    """
+    if not result.file_to_analysis_info:
+      return self._maximum
+
+    minimum = min(analysis_info.min_distance
+                  for analysis_info
+                  in result.file_to_analysis_info.itervalues())
+    if self._maximum is None:
+      return minimum
+
+    return min(float(self._maximum), float(minimum))
+
+
+class LinearMinDistanceFeature(MinDistanceFeature):
+  """Return the minimum distance scaled linearly between 0 and 1.
+
+  That is, when the minimum distance is 0 we return 1; when it is greater
+  than the ``maximum`` passed to the constructor, we return 0. And in
+  between we return values linearly interpolated between those points.
+
+  In principle this normalization isn't strictly required, as the weight
+  of this feature can be be scaled to account for the normalization.
+  However, by normalizing things we ensure that the feature's weight is
+  independent of ``maximum``, which helps training.
+  """
+  def __init__(self, maximum=None):
+    """
+    Args:
+      maximum (float): An upper bound on the return result. This
+        argument is optional and defaults to ``DEFAULT_MAXIMUM``.
+    """
+    if maximum is None:
+      maximum = DEFAULT_MAXIMUM

[inferno, 2016/12/06 18:07:06]

nit: you can just do this in contructor
def __init__(self, maximum=DEFAULT_MAXIMUM)

[Sharu Jiang, 2016/12/06 20:49:19]

I remember in this way, pylint will complain.

+    super(LinearMinDistanceFeature, self).__init__(maximum)
+
+  def __call__(self, result):
+    min_distance = super(LinearMinDistanceFeature, self).__call__(result)
+    return (self._maximum - min_distance) / self._maximum
+
+
+class SquaredMinDistanceFeature(LinearMinDistanceFeature):
+  """Return the minimum distance scaled quadratically between 0 and 1.
+
+  This feature together with ``LinearMinDistanceFeature`` (and a
+  constant feature) allow us to capture any quadratic polynomial of the
+  ``MinDistance``. That is, suppose we had a single feature ``c2*x**2 +
+  c1*x + 1`` with weight ``w``. Rather than using that feature directly
+  (which would require us to specify the hyperparameters ``c2`` and
+  ``c1``) we can instead use three features: ``w2*(x**2) + w1*x + w0``;
+  which enables us to avoid specifying the hyperparameters, by pushing
+  them into the weight parameters instead.
+  """
+  def __call__(self, result):
+    linear_min_distance = (
+        super(SquaredMinDistanceFeature, self).__call__(result))
+    return linear_min_distance * linear_min_distance